Wednesday, February 06, 2008Ammonia in Dark CloudsIn 1969, astronomers discovered that ammonia (NH3) was present in large quantities in interstellar gas clouds. The species was most apparent in regions of star formation where the density and temperature of the gas enabled it to emit bright radio-wavelength radiation. Since then, ammonia has become one of the staple diagnostic probes of the regions where new stars are forming. The processes by which gas and dust condense into new stars is poorly understood; turbulence can inhibit collapse, for example, but exactly how it does so is still a puzzle. Since our sun also must have formed out of the interstellar gas, modeling the details of the processes will help scientists better understand why the sun has the properties it does.

Infrared dark clouds (IRDCs) are dark patches in the sky seen against the continuous, bright infrared background produced by our galaxy. IRDCs are rich in molecules and relatively dense, cool gas, and they are natural sites for future star birth. Studies of IRDCs to date have emphasized those candidates that already to have star formation underway within them. Two SAO astronomers, Qizhou Zhang and Thushara Pillai, together with their graduate student Yang Wang and two colleagues, decided to study an IRDC that does not appear to have strong star formation, and to use ammonia as the key tool to probe its conditions.

The scientists picked an elongated IRC containing about one thousand solar-masses of material and located about fifteen thousand light-years away. Preliminary studies had found evidence for some large clumps in the cloud that might someday evolve into stars. The team used ammonia observations to confirm the presence of a dense clump at one end of the IRDC, a clump that, they conclude, seems already have begun the process of forming a few massive new stars. But in addition, the astronomers report discovering a modest, compact core in the other portion of the cloud where the ammonia reveals very low levels of turbulence. This part of the cloud has ample material to form new stars, and the absence of the disruptive effects of strong turbulence suggests that this region is much younger than even the young stars in the bright clump, and perhaps one of the earliest stages yet identified in the birth of stars. The result reinforces the idea that turbulence is the key to understanding the birth of new stars, and identifies a very young new source for further study.